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Fwd: Re: floor deflections

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Jeff Smith wrote:
> My experience in floor performance is that even for spans greater than 14',
> a L/480 deflection criteria is not desirable especially if there is not
> sheet rock on the ceiling or any loads on the floor such as furniture to
> dampen the vibration. A  while back I specified 9-1/2" TJI/25 at 16" o.c.
> for a 14' span which exceeds L/480 and in hindsight I felt it was a bit
> bouncy. For custom homes in particular, this is an issue. An owner may
> never know the difference unless it is brought to their attention, then it
> will be considered unacceptable and they will do the old
> "jumping-up-and-down-test" through out the house. Trus Joist will provide
> performance crtiteria for their longer spans, because even if the strength
> and deflection is well within the code and table limits, the floor will
> vibrate. The TJI tables can be misinterpreted to allow an inexperieced
> engineer to specify a poor performing, but legal floor for average to very
> long spans. AISC has commentary that empirically limits vibration based on
> the depth to span ratio for steel members.
> Regards,
> Jeff Smith. S.E.

I've been using the recommendations set forth in the paper "A Proposed
Rule of Thumb for Controlling Annoying Vibrations in Residential Floors"
by Frank E. Woeste with some success. The premise is that long span wood
joist system peform best when the fundamantal frequency of vibration is
held below 15 Hz. The formula developed considers the dead load as well
as the EI of the joist. BTW, I believe copies of this paper were offered
some time back on this list serve but I could be wrong.

With regards to Trus Joist. They apparently are updating their design
program to give better advice regarding control of deflections. In
talking with them recently about a project, their engineer apparently
had a pre release of the software which advises about relative
performance levels for various "systems" including multiple blocking
lines, gypsum board ceilings etc. 

I would encourage better communication about wood floor performance
since we all have something to learn. Now if I can just convince TJI
that windows 95 isn't the only operating system alive.......

Barry H. Welliver
Draper, UT

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Richard Lewis, P.E.
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